First international comparison of terahertz laser power measurements

Fig. 1:Happy faces after the work is done in the THz lab of PTB: John Lehman, Andreas Steiger, Qing Sun, and Yuqiang Deng (from the left to the right)

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Measurements of terahertz (THz) power are important for a variety of applications for climate research, security and communication technology, medicine, and manufacturing. The first international comparison of THz laser power measurements took recently place at PTB in Berlin. Andreas Steiger hosted the comparison with the visiting scientists Yuqiang Deng and Qing Sun from the National Institute of Metrology (NIM) of China and John Lehman of the National Institute of Standards and Technology (NIST) who is presently visiting PTB as a Research Fellow of the Alexander von Humboldt Foundation of Germany. Measurements were carried out at two frequencies, 2.52 THz and 0.762 THz and at a power level of approximately 3 mW. The measurement results from each participant agreed very well with the reference value and to each other within the stated uncertainties.

The pilot comparison was undertaken in a relatively short period from May 4 to 8, 2015. Typically, such key comparisons may last years. Organized in a new manner, the participants met in Berlin to compare their standards at one place at one time. The measurement infrastructure such as the laser source and other instrumentation provided by PTB were critical to meeting the proposed schedule.

The comparison was performed via the spectral responsivity of the three different national THz standard detectors used according to the guidelines provided by the Consultative Committee on Photometry and Radiometry (CCPR). The results have still to be approved by the CCPR and will then be published for a larger community. The work represents the first key comparison ever undertaken in the far-infrared spectral region. It represents a milestone which will greatly benefit commercial development of instrumentation and sensors for remote sensing, THz imaging, high-speed telecommunications and time-domain spectroscopy.

Contact:

A. Steiger, 7.34, e-mail: Andreas.Steiger(at)ptb.de

Piece of cake: Arrays of long nanotubes may help measure terahertz laser power

Terahertz radiation can penetrate numerous materials—plastic, clothing, paper and some biological tissues—making it an attractive candidate for applications such as concealed weapons detection, package inspection and imaging skin tumors. However, to date there is no standard method for measuring the absolute output power of terahertz lasers, one source of this type of radiation. Now, researchers at the National Institute of Standards and Technology (NIST) have found that dense arrays of extra-long carbon nanotubes absorb nearly all light of long wavelengths, and thus are promising coatings for prototype detectors intended to measure terahertz laser power.*

The research is part of NIST's effort to develop the first reference standards for calibrating lasers that operate in the terahertz range, from the far infrared at wavelengths of 100 micrometers to the edge of the microwave band at 1 millimeter.

"There is no measurement traceability for absolute power for terahertz laser sources," NIST project leader John Lehman says. "We have customers asking for the calibrations. This coating looks viable for terahertz laser power detectors."

The coating, called a VANTA (vertically aligned carbon nanotube array), has several desirable properties. Most obviously, it is easy to handle. The nanotubes are tens of micrometers to over a millimeter long, so a dense layer is visible without a microscope. A chunk of VANTA can be cut, lifted, and carried like a piece of cake, making it easy to transfer from a silicon surface where the tubes are grown to a laser power detector.

Most importantly, the coating is very dark.






 The NIST team evaluated three VANTA samples with average lengths of 40 and 150 micrometers and 1.5 millimeters (mm) and found that longer tubes reflect less light. The 1.5 mm version reflects almost no light—just 1 percent at a wavelength of 394 micrometers. This result, the first-ever evaluation of a VANTA's reflectance at that terahertz wavelength, indicates that virtually all arriving laser light is absorbed, which would enable highly accurate measurements of laser power.

The 1.5 mm VANTA absorbs more light than comparable coatings such as gold black, but more work is needed to calculate uncertainties and determine effects of factors such as light angle. The project extends NIST's long history in laser power measurements and Lehman's recent advances in ultradark nanotube coatings.**

VANTAs also have desirable thermal properties. NIST researchers found that the material absorbs and releases heat quickly compared to other black coatings, which will make the detectors more responsive and quicker to produce signals. Otherwise, a coating thick enough to absorb long wavelengths of light would not efficiently transmit heat to the detector.

In developing the capability for terahertz laser radiometry, NIST is building a terahertz laser designed for routine measurements and a detector called a thermopile to measure the laser's power. This simple detector design produces a voltage when heat is applied to a junction of two dissimilar metals. NIST researchers used the VANTA to coat a prototype thermopile. Further research is planned to design detectors that might be used as reference standards.

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* J.H. Lehman, B. Lee and E.N. Grossman. Far infrared thermal detectors for radiometry using a carbon nanotube array. Applied Optics. Posted online July 18, 2011. 
** See NIST Tech Beat article "Extreme Darkness: Carbon Nanotube Forest Covers NIST's Ultra-dark Detector," August 17, 2010, at www.nist.gov/pml/optoelectronics/dark_081710.cfm.